The present invention relates generally to circuitry used to obtain rectified signals in any application requiring the derivation of such signals from AC signals.
Generally, the function of a rectifier circuit, as contemplated herein, is to take a signal which has both positive and negative varying amplitudes about some reference, and convert that signal into one which has only positive varying amplitudes about the same or other reference. If the circuit preserves the positive signal and also mirrors the negative signal to be positive with respect to the reference, it is called a full-wave rectifier. If the circuit preserves only the positive signal and deletes the negative signal, it is called a half-wave rectifier. Examples of a sinusoidal input waveform, and ideal half-wave and full-wave rectified waveforms are shown in FIG. 1(a), FIG. 1(b), and FIG. 1(c), respectively.
The general principle in a rectifier circuit is to first compare the input signal with a reference signal. If the input signal is positive with respect to the reference, then it is simply passed through to the output of the circuit. If the input signal is negative with respect to the reference, then it is inverted with respect to the reference and passed through to the output in the case of a full-wave rectifier. In the case of a half-wave rectifier, the negative signal is simply deleted or blocked from passing through to the output of the rectifier. FIG. 2(a) shows a conventional half-wave rectifier circuit (Rectifier A) which is based on this principle, using metal-oxide-semiconductor field effect transistor (MOSFET) devices in the rectifying stage. The circuit includes a transistor M1 which has the input voltage (Vin) applied to the gate, a transistor M2 which has the reference voltage (Vref) applied to the gate, and a transistor M3 which has a bias voltage Vbias applied to the gate. The transistors M1 and M2 are coupled at the source and drain. The common drain is connected to the positive supply. The common source is connected to the drain of the current source FET (M3). The current source FET (M3) ensures that the sum of currents in M1 and M2 remains a constant, independent of Vin and Vref The output voltage (Vout) is measured at the common source node of M1 and M2.
The conventional circuit illustrated in FIG. 2(a) (and as described in more detail in Z. Wang, xe2x80x9cFull-wave rectification that is performed in current domain and very suitable for CMOS implementationxe2x80x9d, IEEE Transactions on Circuits and Systemsxe2x80x94I: Fundamental Theory and Applications, vol. 39, no. 6, pp. 456-463, June 1992), operates on the principle that the source of a FET tracks the drain voltage if the device remains biased in the saturation region of its operation. As Vin approaches Vref and crosses it, there is a region of transition when both devices M1 and M2 are on. This transition region affects the performance of the rectifier adversely. The extent of this region depends on the gain in devices M1 and M2.
In certain semiconductor technologies, particularly those with minimum feature sizes of 0.2 micrometer or less, the field effect transistors (FETs) have low transconductance (a measure of gain through the device) for low values of the gate overdrive voltage. FIG. 2(b) is the simulated output waveform when the sinusoidal input shown in FIG. 1(a) is applied to the circuit schematic shown in FIG. 2(a), the circuit having been designed in a deep submicron complementary metal-oxide-semiconductor (CMOS) semiconductor technology. The MOSFET device performance in this technology is typical of those that have been optimized for performance in digital circuits. As the waveform in FIG. 2(b) shows, the half-wave rectifier does not adequately block the negative-going part of the waveform. This is because the low gain in the MOSFETs M1 and M2 cause them to turn off very slowly when the input sinusoidal waveform is applied.
A need has thus been recognized in connection with overcoming the problem described above and with designing rectifier circuits using FETs or other similar devices with low gain.
In accordance with at least one presently preferred embodiment of the present invention, a circuit arrangement is contemplated for rectifying alternating current (AC) signals in solid-state technologies that have gain devices with low transconductance. A contemplated circuit uses operational amplifiers connected in feedback mode to enhance the effective transconductance of the gain devices.
In one aspect, the present invention provides a rectifier circuit comprising: a current source; at least one transistor connected with the current source; and an operational amplifier connected with the at least one transistor.
For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims.